Radio Frequency (“RF”) device, such as RF identification device, has become an important implementation of an Automatic Identification technique. The object of any RF system is to carry data suitable transponders, generally known as tags, and to retrieve data, by machine-readable means, at a desired time and place to satisfy particular application needs.
Data within a tag may provide identification for an item in manufacture, goods in transit, a location, the identity of a vehicle, an animal, individual, etc. Additional data may be provided for supporting applications through item specific information or instructions immediately available on reading the tag. For example, the color of paint for a car body entering a paint spray area on the production line, the set-up instructions for a flexible manufacturing cell or the manifest to accompany a shipment of goods. In other cases, information may be obtained, indirectly, merely by observing the characteristics of the tags' transmissions—signal strength, multipath delay profile, or time of arrival, for example. RF technologies vary widely in frequency, packaging, performance, and cost.
FIG. 1 shows a conventional RF system 1 which includes an RF tag 10 (e.g., an RFID tag) and an RF arrangement 2 for reading or interrogating the tag 10. The arrangement 2 communicates with a data processing unit 3 to exchange data with the tag 10. The system 1 may also include a facility (not shown) for entering or programming data into the tag 10, if this is not undertaken at source by the manufacturer of the tag 10.
Communications of data between the tag 10 and the arrangement 2 may be performed using a wireless communication technology. Two methods distinguish and categorize RF systems: a first method is based upon close proximity electromagnetic or inductive coupling; and a second method is based upon propagating electromagnetic waves. Coupling is via ‘antenna’ structures 4, 12 forming an integral feature in both the tag 10 and the arrangement 2. While the term “antenna” is generally considered more appropriate for propagating systems, it is also loosely applied to inductive systems.
Transmitting data is subject to the influences of the media or channels through which the data has to pass, including the air interface. Noise, interference and distortion are the sources of data corruption that arise in practical communication channels that must be guarded against in seeking to achieve error free data recovery. The nature of the data communication processes requires attention to the form in which the data is communicated. Structuring the bit stream to accommodate these needs is often referred to as channel encoding and, although transparent to the user of an RF system, the coding scheme applied appears in system specifications. Various encoding schemes can be distinguished, each exhibiting different performance features.
To transfer data efficiently via the air interface or space that separates the two communicating components requires the data to be superimposed upon a rhythmically varying (sinusoidal) field or carrier wave. This process of superimposition is referred to as modulation, and various schemes are available for this purposes, each having particular attributes that favor its use.
RF tags may be either active or passive. A passive tag has a limited utilization (e.g., a theft prevention device in a store). On the other hand, an active tag may provide a plurality of data which can be modified. FIG. 1 shows a conventional active tag 10 including a transmitter or transceiver arrangement 12 which can send to the arrangement 2 data identifying information, current status, current location, or other useful data. In addition, the tag 10 has a battery and may also include a microprocessor 14 and a memory 18. In some cases, the data stored on the memory 18 maybe rewritten and/or modified. However, a disadvantage of the active tag 10 is that the RF transmissions require a significant amount of power. This power requirement results in the additional cost of self-contained battery and the limited operational life of the battery leads to the expense of frequently replacing the batteries and/or the tags themselves.